When a clinker cement is placed in contact with a porous geological formation, an aqueous element of the clinker tends to penetrate the pores of the formation. This is generally referred to as filtration. This quantity which is lost from the clinker compound is referred to as filtration loss or filtrate. It is desirable to limit this loss, particularly of water, from a clinker, since any such loss will alter its physical-chemical properties, for example its rheological properties. The characteristic which allows it to be pumped may be seriously affected or its setting capacity might be altered due to a decrease in the amount of water needed to bring about the various chemical reactions causing it to set and these factors are clearly detrimental to the cementing operation in the well as well as to the mechanical properties of the cement once it has set. This filtration through a porous formation also incurs the risk of destabilising the walls of the well or clogging geological production or potential production zones. Additives specific to clinker cements which act as filtrate reducers, are therefore incorporated with the clinker cements as a means of restricting filtration, for example by reducing the filtration speed, especially by reducing the permeability of the cake formed on the well walls.
Clearly, these filtrate reducer products used with clinker cement will depend on the composition of the clinker in terms of how high the solid content is and in view of the fact that the clinker by definition undergoes a physical-chemical change along the formation during the setting process. In addition, it is desirable that the cake formed due to filtration will do virtually nothing to damage the mechanical bond of the cement with the wall of the well.
Numerous filtrate reducer products for clinker cements are known in the profession. Finely divided mineral particles may be used, for example, or water-soluble polymers such as the sulphonated derivatives of polyacrylamide or HEC (hydroxyethyl cellulose). However, the polymers used in the profession are very sensitive to the high temperatures which can be encountered at subsoil level, and thus lose their initial properties.